A scene where an ultrasonic diagnostic apparatus diagnosis is applied to medical diagnosis has been increasing, because it has feature of being able to perform observation of body tissues in a non-invasive manner and on a real-time basis. In such an ultrasonic diagnostic apparatus, there has been known a pMUT (Piezoelectric Micromachined Ultrasonic Transducer) configured to vibrate, like a drumhead, a diaphragm having a unimorph structure in which a piezoelectric member made of PZT or the like is formed on a substrate, to thereby perform transmitting and receiving of ultrasound.
As compared to a transducer using a piezoelectric member obtained by dividing a bulk PZT material into pieces by dicing, the pMUT-type ultrasound probe has advantages of being able to broaden a frequency band, and promote miniaturization of an element to achieve higher resolution, and of being suited to achieving a two-dimensional array of diaphragms (vibrators) for acquiring a three-dimensional image, and applying to an ultrasound endoscope because of its ability to promote reductions in size and thickness. On the other hand, in a one-dimensional array of diaphragms, an acquirable image is limited to a tomographic image, and thereby a false-negative result is likely to occur depending on operation, so that an operator (medical doctor, ultrasound diagnostic technician) is required to have a certain level of proficiency. For this reason, there is a high need for a two-dimensional array type ultrasound probe capable of acquiring a three-dimensional image.
This type of ultrasound probe is configured to perform energy conversion in the following manner. During transmitting, it is operable to convert electric energy to mechanical energy (vibration of diaphragms), and further convert the mechanical energy to acoustic energy (ultrasound). During receiving, it is operable to convert acoustic energy (ultrasound) to mechanical energy (vibration of diaphragms), and further convert the mechanical energy to electric energy.
In the conversion between mechanical energy and acoustic energy, acoustic matching is important, and it is a point for design to match an effective acoustic impedance of the pMUT with an acoustic impedance of a living body. In the conversion between electric energy and mechanical energy, it is important to enhance energy conversion efficiency of a diaphragm structure including a piezoelectric member. A piezoelectric member becomes most efficient (in an index representing performance of a piezoelectric member, k-value: electromechanical coupling coefficient becomes higher), when utilizing a strain in the same direction (33 direction) as a direction of electric field, so that an ultrasound probe advantageously employs a configuration using a strain in the 33 direction. This configuration allows an inter-electrode distance to be set to a relatively large value, as compared to a configuration in which each electrode is placed to extend in a thickness direction of a PZT-based piezoelectric member, so that there is a merit of being able to improve sensitivity (output voltage to unit pressure) during receiving of ultrasound.
As this type of ultrasound probe utilizing a strain in the 33 direction, there has been known an element having a structure illustrated, for example, in FIG. 17. This element has a thin film-shaped diaphragm d which comprises: a ring plate-shaped piezoelectric member a; a ring-shaped plus electrode b placed on an upper surface, i.e., one surface in a thickness direction, of the piezoelectric member a, at a position radially inward of the piezoelectric member a; and a ring-shaped minus electrode c placed radially outward of the plus electrode b, and the diaphragm d is held by a holding member e. When the element having this structure is driven, electricity is supplied to the pair of plus electrode b and minus electrode c to thereby radially apply an electric field in the 33 direction which is a direction (radial direction) perpendicular to the thickness direction of the piezoelectric member a. Thus, in this element, the piezoelectric member a is strained in the radial direction (deformed in the 33 direction), and, according to this unimorph effect, the diaphragm d is bendingly deformed like a drumhead, to thereby transmit ultrasound.
As the type utilizing a strain in the 33 direction, a micro-shell transducer is disclosed, for example, in the following Patent Literature 1. In this transducer, a piezoelectric member (solid electro-active medium) is laminated on a holding support substrate in such a manner as to form a diaphragm (arched section) inside two shoulder sections, and a pair of electrodes are mounted, respectively, on the shoulder sections, wherein a chamber is formed between the diaphragm and the holding substrate. Then, a voltage is applied along a direction from a plus one of the electrodes to the other, minus, electrode, and thereby an electric field is generated in the same direction. Thus, in this transducer, a stress is induced in the piezoelectric member in the same direction, and, as a result, the diaphragm is moved upwardly or downwardly in a thickness direction thereof, i.e., bendingly deformed in a curved shape.
Meanwhile, in such a diaphragm, if an orientation direction of a piezoelectric member is not coincident with a poling direction, it is hard to expect improvement in piezoelectric properties.
In the structure illustrated in FIG. 17 where the ring-shaped electrodes are placed to utilize a stress in the 33 direction, i.e., a direction perpendicular to the thickness direction, when a poling treatment is performed by applying a relatively strong voltage between the pair of electrodes, a poling direction is radially oriented, so that an orientation direction of the unpoled piezoelectric member a becomes different from the poling direction, resulting in difficulty in improving piezoelectric properties.
More specifically, for example, when the piezoelectric member a is formed on a single-crystal substrate having lattice constants close to those of the piezoelectric member a, the piezoelectric member a can be formed to have a single orientation along one of the thickness direction thereof and a direction perpendicular to the thickness direction (e.g., along the V-V direction in FIG. 17). Even using such a piezoelectric member a having a single orientation, a piezoelectric device utilizing a piezoelectrically-induced stress in a 31 direction, i.e., the thickness direction, can have highly-improved piezoelectric properties, because the orientation direction becomes identical to a poling direction, whereas, in the ring-shaped electrodes placed to utilize a stress in the 33 direction, i.e., a direction perpendicular to the thickness direction, an area where the orientation direction of the piezoelectric member a is different from the poling direction (radial direction) increases, resulting in difficulty in improving piezoelectric properties.
In the Patent Literature 1, a relationship between an orientation direction and a poling direction is not disclosed at all, and it cannot be exactly said that the transducer has excellent piezoelectric properties.